1. Field of the Invention
The present invention relates to a backlight unit and a liquid crystal display device including the same.
2. Description of the Related Art
FIG. 70 shows the schematic structure of a conventional liquid crystal display device. The liquid crystal display device has a liquid crystal display panel 110 and a backlight unit. The backlight unit is provided with a light guide plate 114 and cold-cathode tubes 116 disposed at two end surfaces opposite to each other of the light guide plate 114. Around the cold-cathode tubes 116, reflectors 117 are disposed which efficiently transmit light into the light guide plate 114. Lens sheets 111 and 112 and a diffuser sheet 113 are disposed between the liquid crystal display panel 110 and the backlight unit. In addition, a reflection sheet 115 is disposed on the back side of the light guide plate 114.
For the backlight unit used in the liquid crystal display device, the side lit backlight unit shown in FIG. 70 and a direct backlight unit are generally used, in which a direct backlight unit includes a light source disposed directly below a liquid crystal display panel. They are separately used in such a way that the side lit backlight unit is used for a liquid crystal display device having a screen size of 20 inches (20-inch diagonal screen) or under, in general, and for a liquid crystal display device particularly requiring a reduced thickness. In both of these configurations, a cold-cathode tube is generally used for the light source. With the single exception of a liquid crystal display device having a small screen size for use in a cellular telephone and a personal digital assistant (PDA), a white LED is used for a light source because it does not need much light quantity and it is the most suited for a reduction in size and weight.
Although the cold-cathode tube is mainly used for a liquid crystal having a larger screen size than a cellular telephone and a PDA, environmental issues are increasingly important in recent years, and it is considered to be undesirable to use a cold-cathode tube using mercury.
As a light source replacing the cold-cathode tube, various light sources such as a mercury-free fluorescent tube and an LED are being developed. Among them, an LED is considered to be a promising next-generation light source. In the side lit backlight unit, when an LED is the light source, configurations are considered in which a plurality of white LEDs are arranged and in which a plurality of single color LEDs in red, green, and blue are arranged. In particular, attention is greatly focused on a backlight unit using the combination of red, green, and blue single color LEDs in that it can be used as a backlight for a field sequential display device that sequentially lights single color LEDs and that can implement wide color reproduction which cannot be realized by white LEDs. However, the liquid crystal display device including this backlight unit has a problem that the colors of individual LEDs can be visually recognized in the area corresponding to the vicinity of the incident plane of a light guide plate. This is because in the vicinity of the incident plane, the light from LEDs in different colors are taken out of the light guide plate as the light is not mixed, and the light immediately enters the liquid crystal display panel.
A backlight having a two-stage light guide plate structure using a sub-light guide plate is disclosed by Lumileds Lighting Company, LLC. (Nikkei Electronics, No. 844, pp. 126 to 127, Mar. 31, 2003) in which red, green, and blue lights are mixed in a sub-light guide plate to form uniform white light and then the light is allowed to enter an upper main light guide plate. A problem with this configuration is that efficiency is greatly decreased because the incident efficiency from LEDs to the sub-light guide plate and the incident efficiency of the sub-light guide plate to the main light guide plate are low. A low efficiency increases input electric power to require measures against heat, causes an increase in size due to a radiation fin, etc. In addition, the LEDs to be used are known to generate an increased cost.
Other related lighting and display devices are disclosed in JP-A-2003-215349 and JP-A-2004-95390.